Circularly polarized dipole antenna



J. E. MARTIN CIRCULARLY POLARIZED DIPOLE ANTENNA July 20. 1965 2 Sheets-Sheet 1 Filed Aug. 28, 1962 V11 i ma Inventor cibhn .EMcmzz'n By hisAzzo/"ney United States Patent 3,196,443 CIRCULARLY POLARIZED DWGLE ANTENNA John E. Martin, Magnoiia, Mesa, assignor to United Shoe Machinery Corporation, Flemington, N.J., a corporation of New Jersey Filed Aug. 28, 1962, Ser. No. 220,061 2 Claims. (Cl. 343 -797) This invention relates in general to antennas for use with electromagnetic wave energy and in particular to an antenna comprising two pairs of orthogonally related dipoles and adapted to radiate and receive circularly polarized electromagnetic energy.

Antenna systems are presently known which employ two pairs of orthogonaily related dipoles and which achieve a circularly polarized field. However, such antenna systems have employed external means, e.g., transmission line sections, for exciting radiating currents on the crossed dipoles in phase quadrature which phase relationship of 90 electrical degrees provides the necessary condition for circularly polarized pattern. The matching and phasing means of such feeding systems, such as line sections, transformers and skirts, presents a difficult problem with respect to coaxial mounting and the avoidance of spurious induced currents which might adversely affect the pattern. Also, of course, it is essential to circular polarization for the dipole currents to be equal, this equality implying that the len th of the two dipoles be essentially equal.

Accordingly, it is an object of the present invention to provide an antenna comprising orthogonally related dipoles which may be fed from a common central feed point at the end of a transmission line for the radiation of a circularly polarized field in which such mechanical problems are minimized.

To this end and in accordance with a feature of the invention, an antenna is provided comprising two pairs of orthogonally related dipoles each comprising oppositely directed arms in which a portion of each dipole adjacent to the central points of feed (herein termed the base portion) comprises a section of transmission line which, while constituting a portion of the dipole itself, provides a suitable series impedance for achieving the desired phase quadrature when the dipoles are fed at their base from the end of a transmission line.

Another object of the present invention is to provide an antenna having a broadly directive substantially cardioid radiation pattern, such a pat-tern being particularly usetul in such applications as require directive systems, for example, as telemetry and communication with space vehicles.

Other features and advantages of the invention will best be understood from the following description taken in connection with the accompanying drawings in which:

FIG. 1 is an angular view of an antenna, embodying the present invention;

FIG. 2 is a functional schematic of the antenna;

FIG. 3 is an enlarged view partly in section of a portion of the antenna looking along the arrow III of FIG. 1;

FIG. 4 is a section taken generally on line IV-IV of FIG. 1;

FIG. 5 is a section taken generally on the line VV of FIG. 1; and

FIG. 6 is a section of a portion of the antenna taken along the line VI-VI of FIG. 3.

Referring to the drawings, the antenna is shown as supported from a housing 10 which may, for example, be a trailing surface of a vehicle, the support comprising a coaxial transmission line 12 extending through a gasket 14 fixed to the housing 10. The transmission line 12 comprises (see FIG. 4) an outer conductor 16 and an inner conductor 18 separated by insulating material 2%. At the 3,196,443 Patented July 20, 1965 outer end of the coaxial line a portion of the insulating material 20 is removed and an annular stud 22 is welded to the center conductor 13 while the corresponding length of the outer conductor is cut away in a 180 sector. To the stud 22 are silver soldered in orthogonal relation two stubs 24 and 26 (FIG. 3) while two other stubs 28 and 3% are similarly secured to the remaining portion of the tip of the conductor 16 the stubs thus extending radially at intervals from a common centrally located feed point at said end of the coaxial line. To the four stubs 24, 26, 28 and 30 are fastened four arms 32, 34, 36 and 33, respectively, separated in each case from the stubs by a thin layer of insulation 40, the fastening means comprising insulating sleeves 42. The arms 32 and 36 with the stubs 24 and 28 constitute one pair of dipoles while the arms '34 and 38 and the stubs 26 and 30 constitute a second pair of dipoles orthogonally related to the first pair.

In one pair of dipoles comprising the arms 32 and 36, the arms and stubs are connected by rivets 44, while in the other pair of dipoles comprising the arms 34 and 38, no such connection is made.

The stubs and the base portions of the arms which coextend with the stubs will be seen to constitute transmission line sections adapted to provide reactive-line impedances .to the flow of dipole current when the dipole is fed by the coaxial line. As shown in FIG. 2, the load presented to the feed points 50, corresponding to the point of connection of the inner ends of the stubs to the coaxial line, comprises two parallel circuits 52, 54, one for each dipole. in each circuit, the radiation resistance R is in series with a reactance which is the net effective reactance of the two line sections of the corresponding dipole.

According to the present invention the line sections of the two orthogonally related dipoles provide in series with the radiation resistance of the dipoles, reactances which as between the two dipoles are of opposite signs, the reactance in one being an inductive reactance +jX while the reactance in the other dipole is a capacitive reactance 'X, the two react-ances being equal at the operating frequency so that the net load at the load points 50, that is, the antenna input impedance, is purely resistive. Preferably, this resistive load will be matched to the characteristic impedance of the coaxial line.

Further to achieve the proper phasing of the antenna currents to produce a circularly polarized field, the effective reactance in each dipole is made equal to the radiation resistance R.

The appropriate values of reactance can be provided by suitable establishment of the parameters of the transmission line sections by appropriate design in accordance with well-known transmission line theory. Transmission line sections when open circuited provide a series reactance X= 'Z cot (,5 and when short circuited a series reactance X 'Z tan d where Z is the characteristic impedance of the line and 5 the electrical length of the section. Further herein, the net series reactance of the two open circuited sections in one dipole is X 'Z cot s and the net series reactance X of the short circuited sect-ions in the other is jZ tan p where Z is defined as the effective characteristic impedance of the line combination and is the electrical length of each section.

The current flowing in the two pairs of dipoles in a given phase for a voltage of the same phase is:

For the inductive dipole:

Assuming that X is made equal to R by suitable ad- 'justment of the parameters Z andthe phase of while the phase of so that the current a-rein quadrature. At the same time,

the ratio of the absolute values of the currents 'while the reactive components'of current are equal and in opposite phase and cancel so that the transmission sees ':a purely resistive impedance.

Accordingly, the conditions requiste to the radiation of a circularly polarized field are present under the recited conditi-ons. r l

of the coaxial .line. It is, of course, not necessary where the antenna is fed by a balanced line. To provide a broadly directive radiation pattern, a pair of directors 58 are attached to the balun 56. and located about 0.1 from A fbalun56is provided at the end'of the coaxial line 12 .to eliminate unbalancingcurrents flowing on the outside arms and stubs.

the driven elements. With the aforedescribed director, the radiation pattern assume-s substantially the shape of a cardioidiof revolution.

Having thus described my invention, what I claim as new and desire to seeure by Letters Patent of the United States is: V v

1. In an antenna, a rigid coaxial trans-mission line, four stubs extending radially at intervals from a'com mon central feed point at'an' end of said line, two' of said stubs being-connected -tO '3.Ch conductor of said line, four radially dis-posed conductive arms of equal length longer than the stubsyeach arm being carried by a stub and extending parallel thereto from its base and separated therefrom by a thin layer of insuIatiOm'thereby forming two pairs of orthogonally related dipoles with a transmission line section at the :base of each arm electrically in series between the arm and a conductor of the coaxial line and means for electrically eonnecting the stubs and arms of tone dipole at a point spaced from the base thereof whereby .at a given operating frequency the input impedence of one dipole across the inner ends of the stubs thereof has a reactive component equal to and of opposite sign 1 to the corresponding input impedance of the other dipole.

2. Apparatus asin claim 1 and addition-ally comprising sleeves of insulating jmaterial snugly encompassing the References Cited by the Examiner :UNITED STATES PATENTS 2,275,030 3/42 Epstein 343 7 98 X 2,823,381 '2/58. Martin 343-797 2,976,534- 3/61 Kampins-ky 343-797 HERMAN KARL SAALBACH, Primary Examiner. 

1. IN AN ANTENNA, A RIGID COAXIAL TRANSMISSTION LINE, FOUR STUBS EXTENDING RADIALLY AT 90* INTERVALS FROM A COMMON CENTRAL FEED POINT AT AN END OF SAID LINE, TWO OF SAID STRUBS BEING CONNECTED TO EACH CONDUCTOR OF SAID LINE, FOUR RADIALLY DISPOSED CONDUCTIVE ARMS OF EQUAL LENGTH LONGER THAN THE STUBS, EACH ARM BEING CARRIED BY A STUB AND EXTENDING PARALLEL THERETO FROM ITS BASE AND SEPARATED THEREFROM BY A THIN LAYER OF INSOLATION, THEREBY FORMING TWO PAIRS OF ORTHOGONALLY RELATED DIPOLES WITH A TRANSMISSION LINE SECTION AT THE BASE OF EACH ARM ELECTRICALLY IN SERIES BETWEEN THE ARM AND A CONDUCTOR OF THE COAXIAL LINE AND MEANS FOR ELECTRICALLY CONNECTING THE STUBS AND ARMS OF ONE DIPOLE AT A POINT SPACED FROM THE BASE THEREOF WHEREBY AT A GIVEN OPERATING FREQUENCY THE INPUT IMPEDENCE OF ONE DISPOLE ACROSS THE INNER ENDS OF THE STUBS THEREOF HAS A REACTIVE COMPONENT EQUAL TO AND OF OPPOSITE SIGN TO THE CORRESPONDING INPUT IMPEDANCE OF THE OTHER DIPOLE. 